The present invention relates to a method and apparatus for the inspection by comparison of various items or products. For example, the invention may be applicable to the inspection of various types of mass-produced industrial items such as printed circuit boards or integrated chips, or may be used for comparison of finger prints or for DNA matching.
In many areas of industry the traditional method of inspecting an item is by comparison with a known master. This inspection is normally carried out by a trained inspector. Depending on the degree of quality control required in a particular field, every item or article may be inspected, or only a certain proportion. Difficulties have arisen in recent years, however, with the growing complexity of many items to be inspected, often accompanied by a reduction in size. The layout of a printed circuit board (PCB) provides a good example of this.
This inevitably makes human inspection harder. Operators need to be more skilled, each individual inspection may take longer and operators will fatigue faster. It is therefore-becoming harder and harder to find simple methods of maintaining a high standard of inspection.
Inevitably therefore research and development has concentrated on finding methods of automating the inspection process. To date, however, the results are often far from satisfactory. Equipment is often expensive and not necessarily any more reliable then traditional human inspection methods. Furthermore automated inspection systems tend to be specific to each task and often lack the flexibility to be used in a wide range of different applications.
Although not close to the present-invention, the following may be identified as being representative of the many forms of inspection apparatus and methods known in the prior art.
U.S. Pat. No. 5,517,235 describes a PCB inspection system in which cameras are used to obtain images of the PCB, which images can be varied in size by use of zoom lenses in accordance with the tolerances of different parts of the PCB. This allows the magnification of the images to be varied as desired.
In U.S. Pat. No. 5,550,853 a method is described for the inspection of items formed of individual arrays. Selected portions of the arrays are captured images which are then oriented in a two-dimensional array with large scale regularity. This large scale regularity, facilitates the inspection and detection of defects.
In U.S. Pat. No. 4,707,734 a system for detecting flaws in PCBs involves scanning the board to be inspected with a video camera to produce an image, digitising that image into a sequence of pixels and then comparing the digitised data with stored data relating to an ideal component.
In many respects human operators remain the most skilled and flexible approach to inspecting or comparing products or articles, however there remains a need to provide a method and apparatus that facilitates human inspection of complex articles.
According to the present invention there is provided a method for human comparison of two articles to identify any differences between them, comprising the steps of:
(a) forming an image of each said article,
(b) optically superimposing said images in a common image plane,
(c) viewing said superimposed images, and
(d) observing any differences between said articles as a three-dimensional component of said superimposed images.
By means of this invention there is provided a particularly fast reliable and simple method of comparing articles. The method may be used in a quality control inspection, for example of a PCB, by comparing one PCB with a PCB known to be correct, or the method may be used for comparison purposes, for example for fingerprint of DNA matching where complex images are to be compared.
The method of the present, invention employs the old technique of stereoscopy. In traditional stereoscopy two images are formed that are similar but not identical. When viewed together the brain detects the differences between the two images but processes the difference so as to give a three-dimensional effect. In the present invention this effect is used in reverse. There is no intention of course that the images be different, they will normally be intended to be the same if, say, a PCB meets its quality standard. But if there is a defect or difference between the two images this will be perceived by a viewer, normally as a three-dimensional effect where the defect may be seen as a xe2x80x9cfloatingxe2x80x9d element.
Different inspectors may perceive this effect slightly differently, but this does not matter. After a short period of training each inspector will know what he or she is looking for in the event of a defect.
The images to be used may be direct images of the objects in question by placing each said article in a respective one of two optical paths, said optical paths carrying images of said articles to respective eyes of a viewer.
Alternatively the xe2x80x9carticlesxe2x80x9d being compared may be video images of the objects themselvesxe2x80x94this would for example allow three-dimensional articles to be compared, and it would facilitate comparison of articles where direct inspection is difficult since it allows remote cameras to be used. In such an embodiment two remote articles are compared by placing an image of each said article in a respective one of two optical paths, said optical paths carrying said images to respective eyes of a viewer.
The invention may in particular be used as a method for defect detection in an article, said method comprising comparing an article to be tested with an article known to be satisfactory. Alternatively the method may be one of simple comparison to find out whether two similar articles are in fact the same, eg two fingerprints.
According to the invention there is further provided apparatus for comparing two articles to identify any differences therebetween, comprising:
means for forming images of said articles, and
means for carrying said images to respective eyes of a viewer such that said images are superimposed.
In a preferred embodiment the apparatus may comprise a support member on which said articles may be supported at predetermined locations, and image forming means located above said support member, said image forming means comprising two optical paths for carrying an image of a respective article supported at a predetermined location to a respective eye. With such apparatus two-dimensional or generally planar articles may be compared directly by locating them on the support member at the predetermined locations.
Alternatively where the articles are three-dimensional, or for some other reason cannot be viewed directly, the apparatus may comprise two cameras for forming images of said articles, and means for displaying said images in respective optical paths, said optical paths carrying said images to respective eyes of a viewer.